Field of the invention
The present invention relates to wireless communication, and more particularly, to a method by which a terminal transmits a synchronization signal for a device-to-device (D2D) operation in a wireless communication system, and the terminal using this method.
Related Art
In International Telecommunication Union Radio communication sector (ITU-R), a standardization task for International Mobile Telecommunication (IMT)-Advanced, that is, the next-generation mobile communication system since the third generation, is in progress. IMT-Advanced sets its goal to support Internet Protocol (IP)-based multimedia services at a data transfer rate of 1 Gbps in the stop and slow-speed moving state and of 100 Mbps in the fast-speed moving state.
For example, 3rd Generation Partnership Project (3GPP) is a system standard to satisfy the requirements of IMT-Advanced and is preparing for LTE-Advanced improved from Long Term Evolution (LTE) based on Orthogonal Frequency Division Multiple Access (OFDMA)/Single Carrier-Frequency Division Multiple Access (SC-FDMA) transmission schemes. LTE-Advanced is one of strong candidates for IMT-Advanced.
There is a growing interest in a Device-to-Device (D22) technology in which devices perform direct communication. In particular, D2D has been in the spotlight as a communication technology for a public safety network. A commercial communication network is rapidly changing to LTE, but the current public safety network is basically based on the 2G technology in terms of a collision problem with existing communication standards and a cost. Such a technology gap and a need for improved services are leading to efforts to improve the public safety network.
The public safety network has higher service requirements (reliability and security) than the commercial communication network. In particular, if coverage of cellular communication is not affected or available, the public safety network also requires direct communication between devices, that is, D2D operation.
D2D operation may have various advantages in that it is communication between devices in proximity. For example, D2D UE has a high transfer rate and a low delay and may perform data communication. Furthermore, in D2D operation, traffic concentrated on a base station can be distributed. If D2D UE plays the role of a relay, it may also play the role of extending coverage of a base station.
Meanwhile, transmission of a synchronization signal may also be required in the D2D operation. The synchronization signal may provide a reference time point in signal transmission/reception, decoding, or the like based on the D2D operation. Conventionally, for a terminal in a radio resource control (RRC) connected state with respect to a network, whether the synchronization signal for D2D operation is transmitted has directly been controlled by the network in an on/off manner. However, this method is based on a premise that a frequency in the RRC connected state is identical to a frequency for the D2D operation.
There is a need for a synchronization signal transmission method for the D2D operation of the terminal when the frequency in the RRC connected state is different from the frequency for the D2D operation.